Eyes of Molluscs aud Arthropods. 605 



do not properly belong liere, and tlierefore only enougli will be said to 

 serve for comparison between tlie developmeut of these fibres and thosc 

 of tlie eye. A more dctailed account of the scuse hair papillae and 

 their orig-iu will be rcscrved for a future paper. 



The sense hair papillae may originate at any place along the outer 

 surface of the velum, or along the optic tract. They appear, at first, as 

 thickeuiug-s of the hypodermis — similar to those of the eyes — which 

 soon become conical, with a tuft of stifi' scuse hairs at the apex (fig. 6). 

 The in ward prolif eratio n ofthecells, atthatpoint,gives 

 rise to an e et od ermi e cor e, which becomestransformedìnto 

 a longitudinal nerve with which every tentacle is pro- 

 vi de d. As the papillae increase in length, tufts of sense hairs are 

 formed ou the sides, each connected with one or two ganglionic cells 

 (fig. 6), In those papillae, on the surface of the velum (fig. 8), which do 

 not develop into tentacles, no nerve is formed; but two or three cells are 

 separated from the summit of the papilla, and wander into the under- 

 lyiug tissue, there formiug ganglionic cells, the nerve-like ends of which 

 may terminate in a small number of sense hairs ; or, if the cells are 

 more highly specialized, the sense hairs may be absent, and the terminal 

 fibres divided into numerous fibrillae, which supply the adjacent cells. 



Many of the papillae are deeply pigmented; others on the contrary 

 may be entirely colorless. 



Ostrea. 



I bave not been able to find, in this genus, the eyes referred to by 

 Will and others, and, since more recent authors bave been equally 

 unsuccessful, we must, I think, believe that he was either mistaken, or 

 examined some other species. 



Besides the pigment scattered irregularly over the surface, I bave 

 observed, beneaththeepidermis, a greatmany pigment balls (from .01 

 to .005 mm in diameter) smaller than, but similar to, those of Cardium. 

 The tentacles are nearly pigmentless and ciliated. The hypodermis at 

 the extremities, is thickened and supports a number of sense hairs; 

 this is also the case with Mactra. In the pigmented areas, the same 

 colorless cells, as in Arca^ are seen surrounded by pigmented ones, and 

 undoubtedly represent scattered ommatidia. 



I can confirm the statement of Sharp, that Ostrea is sensitive to 

 ehanges in the intensity of light. He has sectioned the pigmented areas 

 and finds two kinds of cells, pigmented and colorless ones, which he 



